There are really two forces at play in determining where you rank with the search engines. The search engines making updates and adjustments to their algorithms and the Internet changing on its own. New sites come on line every day and every day we add or change the content on existing sites. The Internet is an amorphous being of information that ebbs and flows all on its own. When these two forces come together, you have constant change.
SEOMoz studied this phenomena last year and reported that more than 80% of search engine results pages (SERPs) change on a daily basis. Daily. That is hard to keep up with. The demands on SEO are greater than ever and are likely to continue barring any unforeseen major change in the way the Internet is managed, accessed and utilized. In fact, Google’s Matt Cutts stated that Google tweaks its algorithm on a daily basis. Engineers make anywhere from 300-500 changes per year.
This adds credence to the statement that SEO requires constant attention and is a long-term commitment. You cannot simply sit back and celebrate getting to the top of the SERP and then move on to other things. You must be diligent and always work to maintain that top position because it can be gone in a flash.
When Pigeon recently swooped in on the industry, it had significant consequences and they are still being felt. These types of change do not require throwing the baby out with the bathwater. They do, however, require study and a swift response to regain any ground lost due to the change.